1. Field of the Invention
The present invention relates to an image heating device for heating an image borne by a recording sheet.
2. Related Background Art
In the image forming apparatus for transferring a toner image on a recording sheet and heating such transferred image to obtain a permanent image, there is conventionally employed an image heating device as shown in FIG. 1.
Referring to FIG. 1, a heating roller 1 is composed for example of a metallic core 11 such as of aluminum or iron, and a releasing resinous layer 12 such as of PFA or PTFE, and is heated internally by means of heater 4. The temperature of the heating roller 1 is detected as the surface temperature thereof by a temperature detecting element 3 maintained in contact with the heating roller 1, and the surface temperature is maintained at a predetermined temperature by intermittent activation of the heater 4 by means of a temperature control circuit (not shown). The temperature detecting element can be positioned within the passing area of the recording sheet in case of the image heating device equipped with cleaning means, but, in case of the image heating device lacking such cleaning means, it is generally provided in a non-image area in order to prevent smear on the image.
A pressure roller 2, rotated in pressure contact with the heating roller 1, is composed of a metallic core 13 such as of aluminum or iron, a heat-resistant elastic layer 14 such as of silicone rubber or silicone sponge of a low hardness provided thereon, and a surfacial covering layer 15 composed of a releasing resinous material such as PFA or PTFE.
A recording sheet P, bearing a toner image T thereon, is guided by an entrance guide 6 to the nip between the heating roller 1 and the pressure roller 2, and is subjected to image fixation under heat and pressure. The entrance guide 6 is generally composed of a controlled resistance material such as PBT (having a resistance of 10.sup.8 to 10.sup.10 .multidot..OMEGA.), or has a metallic guide surface such as of stainless steel and employs the above-mentioned controlled resistance material at the junction with a fixing frame. This is to avoid drawbacks such as toner scattering, caused by electrostatic charging of the guide surface resulting from the friction contact with the recording sheet if the entrance guide is composed of an insulating material. Also in order to avoid the generation of crease in the recording sheet P in the passing thereof through the nip, it is customary to provide the heating roller 1 and the pressure roller 2 with adequate inverse crowning in the longitudinal direction thereof and to adequately adjust the position of entry of the recording sheet into the nip between the heating roller and the pressure roller, by means of the entrance guide 6.
In such an image heating device, the thickness of the heating roller 1 is often made equal to or less than 1.0 mm, in order to reduce the heat capacity of the heating roller, thereby shortening the warm-up time. In such structure, if only one heater is employed, there is encountered an excessively high temperature in the non-passing area of the recording sheet, particularly in case of printing with small-sized sheets. Particularly in a high-speed apparatus, the printing speed has to be significantly lowered in such printing with small-sized sheets.
For avoiding such a drawback, there is proposed a configuration employing two heaters of different heat distributions. FIG. 2 shows the cross-sectional structure of such configuration, and FIG. 3 shows the heat distribution of the heaters and the arrangement of segments. The illustrated heat distribution of the heaters is designed for sheet transportation with the reference position at the center. A heater 4a is used for the printing of a small-sized sheet, and has heat distribution in a portion where the heat is absorbed by the sheet. The heater 4b is used, in combination with the heater 4a, for the printing of a large-sized sheet. FIG. 13 shows the heater lighting ratio for different sheet sizes. For the lighting of the heater 4a for 500 msec., the heater 4b is turned on for 500 msec in case of printing an A3-sized sheet, 300 msec in case of printing a B4-sized sheet and 100 msec in case of printing a longitudinally-oblong A4-sized sheet. Such lighting ratios are generally so selected as to obtain a substantially flat temperature distribution on the heating roller, for the sheets of a most frequently used weight range of 65 to 80 g/m.sup.2.
In the image heating device of the above-explained configuration, because of the limited heat capacity of the heating roller, the temperature distribution on the heating roller becomes different because of the difference in the heat amount carried away by the sheets, depending on the weight thereof. In the continuous printing operation, the temperature distribution (at the thirtieth sheet or thereafter) assumes the form shown in FIG. 4, and, even for the sheets of a same size, the image fixing ability may become deficient by the temperature decrease in the central area, particularly in case of thick recording sheets. This is because the longitudinal heat conduction in the metallic core of the thin heating roller cannot match the supplied heat amount. A similar phenomenon may be caused by a lowered voltage of the power supply. For example a lowering by 15% of the power supply voltage reduces the output of the heater to 78% of the rated power, so that the output of a heater of 800 W is reduced to 623 W. With such lowering of the heater output, the image fixing ability may become deficient at the central area, even for a sheet weight of about 90 g/m.sup.2.
Also because of recent wide variety of sheet materials, it is also required to pass a thick sheet such as 128, 160 or 200 g/m.sup.2, and the fixing ability may become deficient because of such heavy sheet weight or the variation in the power supply voltage.